Although transistors formed using amorphous silicon had been used for conventional display devices typified by liquid crystal televisions, an oxide semiconductor has attracted attention as a material which replaces a silicon semiconductor in order to form transistors. For example, an active matrix display device, in which an amorphous oxide including In, Ga, and Zn is used for an active layer of a transistor and the electron carrier concentration of the amorphous oxide is lower than 1018/cm3, is disclosed (see Patent Document 1).
However, some problems of a transistor formed using an oxide semiconductor have been pointed out. One of these is stability of characteristics, and it is pointed out that threshold voltage is changed by irradiation with visible light and ultraviolet light (e.g., see Non-Patent Document 1). Further, a problem, in which characteristics are changed by a bias-temperature stress test in terms of reliability of a transistor, has been pointed out (e.g., see Non-Patent Document 2).